The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is outstripping system capacity. If this trend continues, the effects of rapid growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as to maintain high quality service and avoid rising costs.
Throughout the world, one important step in cellular systems is to change from analog to digital transmission. Equally important is the choice of an effective digital transmission scheme for implementing the next generation of cellular technology. Furthermore, it is widely believed that the first generation of Personal Communication Networks (PCN), (employing low cost, pocket-size, cordless telephones that can be carried comfortably and used to make or receive calls in the home, office, street, car, etc.), would be provided by the cellular carriers using the next generation digital cellular system infrastructure and the cellular frequencies. The key feature demanded in these new systems is increased traffic capacity.
In mobile cellular radio telephone systems using time-based multiple access methods such as TDMA or CDMA, it is often necessary for mobile transmitters to employ an appropriate transmitter power dependent on their distances from the base station and the proper transmitter timing associated with their various propagation delays to the base station. Proper transmitter power selection maintains all mobile transmitter signals received at the base station at approximately the same level to avoid excessive level differences which can result in interference by stronger signals.
In a TDMA system, timing of the mobile transmitter signals is controlled as a function of distance from the base station to ensure that signals arrive at the base station in their correct, assigned time-slot, and do not overlap. In a CDMA system, timing is controlled to reduce the width of the timing uncertainty region over which the code-correlating receiver must search, particularly when the mobile station begins transmitting. In both types of systems, once a mobile station achieves duplex communication with a base station, information transmitted from the base station can continuously control the mobile station transmitter's power and timing.
In CDMA applications, power and timing accuracy are most difficult to maintain. Maintaining the correct power level is paramount when signals overlap in time and frequency. The signals may be separated at the receiver by correlating the received signals with the corresponding despreading codes if the differences in power levels are not substantial.
In a conventional CDMA receiver, the suppression of unwanted signals in the correlation process is limited by the so-called processing gain. If an unwanted signal exceeds a desired signal by an amount greater than the processing gain, the desired signal cannot be decoded.
In U.S. Pat. Nos. 5,151,919 and 5,218,619, both, entitled "CDMA Subtractive Demodulation" by the present inventor, systems are described in which all signals are decoded at a base station in the order of strongest to weakest signals. The stronger signals are subtracted from the composite signal after decoding and before demodulating the weaker signals. As a result, greater level differences can be tolerated when the receiver knows what signals are present and their signal strength ranking. However, even the innovative subtractive CDMA system has difficulty with the sudden, unexpected appearance of a new signal at an arbitrary signal level.
The present invention overcomes the problems of the prior art by employing a method whereby a mobile station can estimate the power and/or timing advance necessary to make a first transmission to a base station.